TDD Time Slot Splitting

ABSTRACT

The invention discloses a method for a cellular communications system, in which traffic is sent in frames, each frame comprising a first number of subframes, with a second number of said subframes being available for at least either uplink or downlink traffic. At least one of said second number of subframes is made to comprise at least three parts, as follows:
         One part which is utilized for uplink traffic,   One part which is utilized for downlink traffic,   One part which is utilized as a guard period,
 
with said guard period part being scheduled between the uplink and the downlink parts. The duration of at least two of said three parts may be varied to fit the current system need.

This application is a continuation of U.S. patent application Ser. No.12/047,921, filed Mar. 13, 2008, the disclosure of which is incorporatedhere by reference.

TECHNICAL FIELD

The present invention discloses a method for use in a cellularcommunications system, in which system the traffic in a cell is sent inframes. Each frame comprises a first number of subframes, and a secondnumber of said subframes is available for at least either uplink ordownlink traffic,

BACKGROUND

In cellular wireless systems, both current systems such as, for example,UTRA (UMTS Terrestrial Radio Access) systems, and future systems such asthe LTE (Long Term Evolution) systems, a principle which is used is socalled TDD, Time Division Duplex, i.e. a principle according to whichuplink and downlink traffic occur during different periods of time, socalled subframes, which are comprised in a larger frame. Usually, in aTDD system, the uplink and downlink traffic use the same carrierfrequency,

Due to the fact that uplink and downlink traffic share one and the samefrequency in a TDD system, interference problems may occur betweendifferent cells in the system. In particular, downlink traffic from onecell may cause interference in other cells.

In addition to interference between different cells in one and the sameTDD system, interference may also occur between different cells indifferent but co-located or adjoining systems, for example systems whichare run by different operators.

One way of reducing the problems of inter-cell interference in TDDsystems is to arrange so called “guard periods” at the transitionsbetween downlink and uplink traffic, i.e. periods during which notraffic may occur. Guard periods may also be arranged at transitionsfrom uplink to downlink.

Co-existence, i.e. the ability to avoid interference between cells inone and the same system, as well as between cells of adjoining orco-located cells of different systems, is an important factor.

However, the frame structures in some current UTRA and LTE TDD standardsoffer limited co-existence opportunities. To make efficient co-existencepossible, the frame structure should preferably make it possible to havea large flexibility when it comes to configuring it to overcomeinterference problems, both interference from cells in the “own” systemas well as from cells in co-located or adjoining cells of other systems.

SUMMARY

As has emerged from the description above, there is thus a need for asolution by means of which increased coexistence in the form of reducedrisk of interference between cells of one and the same or differentwireless cellular systems can be increased, in particular in systemswhich use the TDD principle.

Such a solution is offered by the present invention in that it disclosesa method for use in a cellular communications system, in which systemthe traffic is sent in frames, with each frame comprising a first numberof subframes and with a second number of subframes being available forat least either uplink or downlink traffic.

According to the method of the invention, at least one of the secondnumber of subframes is made to comprise at least three parts, asfollows:

-   -   One part which is utilized for uplink traffic,    -   One part which is utilized for downlink traffic,    -   One part which is utilized as a guard period.

The guard period part is placed between the uplink and the downlinkparts, and according to the invention, the duration of at least two ofsaid three parts may be varied to fit the current system need.

Thus, the present invention offers a solution by means of which asubframe may be made to comprise a guard period of varying length, sothat it can be suited to overcome the interference problems of aspecific system, and so that the remaining part of the subframe can bemade to comprise uplink and downlink traffic in varying proportions,since, according to the invention, the remaining part can be dividedbetween the uplink and downlink directions, thus ensuring maximumefficiency regarding the use of the available resources.

The inventive method can in one embodiment be applied to a system whichuses so called TDD, Time Division Duplex, i.e. a system with a so calledunpaired spectrum, in which uplink and down link traffic in at least afirst plurality of cells in the system occur during different subframes,but on the same frequency. In another embodiment, the inventive methodmay also be applied to a system which uses so called half duplex FDD,Frequency Division Duplex, so that uplink and down link traffic for oneand the same user in at least a first number of cells in the systemoccur during different subframes, and on different frequencies.

Suitably, a subframe of the invention is placed at a transition betweenuplink and downlink, either at a transition from downlink to uplink, orat a transition from uplink to downlink.

The invention also discloses a transceiver for use as a controlling nodein a cell of a system of the invention, and a transceiver for use as auser terminal in a system of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, withreference to the appended drawings, in which

FIG. 1 shows a schematic view of a system in which the invention may beapplied, and

FIGS. 2 and 3 show prior art frames, and

FIG. 4 shows a subframe of the invention, and

FIG. 5 shows a flow chart of a method of the invention, and

FIG. 6 shows a block scheme of a first transceiver of the invention, and

FIG. 7 shows a block scheme of a second transceiver of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a schematic view of a wireless cellular system 100 in whichthe present invention may be applied. The invention will in thefollowing be described with terms from so called LTE systems (Long TermEvolution), but it should be pointed out that this should not beconstrued as limiting the scope of protection sought for the presentinvention, the LTE terminology is merely used in order to facilitate thereader's understanding of the present invention, the invention can beused in other types of wireless cellular systems as well.

In addition, the word “traffic” is used in this text. It should bepointed out that the word “traffic” in this text is taken to mean allcommunication that is sent in downlink and uplink, e.g. both so called“payload data” and control signals, etc.

Returning now to the system 100 shown in FIG. 1, the system comprises anumber of cells one of which is shown as 130 in FIG. 1. A cell in thesystem can accommodate a number of users, one of which is shown as 120in FIG. 1, and, using LTE terminology, the user is shown as a “UE”, UserEquipment.

In the system 100, there is also a controlling node, shown as 110 inFIG. 1, which has as one of its functions to control the traffic to andfrom the UEs 120 in the cell 130. The controlling node is in LTE knownas “eNodeB”, evolved NodeB.

Traffic from the UEs 120 to the eNodeB 110 is known as uplink traffic,UL, and traffic from the eNodeB to the UEs is known as downlink traffic,DL.

Both UL and DL traffic is sent in so called frames, and an LTE TDDsystem at present has two different frame structures, known as type 1and type 2. Type 1 will be described with reference to FIGS. 2 and 3: asshown in FIG. 2, one frame of Type 1 comprises 10 so called subframes,SF, shown as 201-210.

As indicated with arrows in the subframes 201-210, a subframe can beused either for DL or UL traffic. However, as has also been indicatedpreviously, in systems in which a multitude of cells are synchronizedwith respect to which subframes that are used for UL or DL, at, forexample, a transition from DL to UL, inter-cell interference can becaused in neighboring cells by “lingering” DL traffic.

Such interference can be reduced, if not eliminated, by means of socalled guard periods, which are periods during which no transmissionsmay be made, and which are created by “silencing” the last part of a DLsubframe, as shown in FIG. 3.

In addition, it can also be mentioned that TDD systems can use a frameof a type known as type 2, which differs slightly from type 1, but whichbasically also uses the principle of guard periods in order to overcomeinterference problems.

It is a purpose of the present invention to offer a new frame structurewhich may be used to replace the existing frames of type 1 and type 2,since it is also desirable to reduce the amount of options, and to haveonly one type of frame.

A basic idea behind the present invention is to let a subframe comprisethree parts, one of which is used for uplink traffic, one of which isused for downlink traffic, and one of which is used as a guard period.Suitably, the “guard period part” is placed between the “uplink part”and the “down link part”.

By means of the invention, as will emerge from the more detaileddescription below, it will be possible for an LTE system which uses TDDor half duplex FDD to coexist in a better way than hitherto with otherLTE TDD systems as well as with 3G TDD systems such as TD-SCDMA, or withWiMax systems.

As opposed to UTRA TDD and LTE TDD systems, in which subframes can beallocated to uplink or to downlink, the present invention makes itpossible to allocate, for example, the first part of a subframe todownlink transmission and the last part of a subframe to uplinktransmission. The DL part of an inventive subframe will be referred toas DwPTS, and the UL part of the inventive subframe will be referred toas UpPTS.

FIG. 4 shows a subframe 420 of the invention, flanked by a DL subframe410 and an UL subframe 420. As shown in FIG. 4, in the inventivesubframe 420, between the two UL/DL periods, i.e. DwPTS and UpPTS, aguard period, GP, of varying length may thus be configured. The durationof the GP of the inventive subframe will be based on a number ofparameters, one of which may be the maximum roundtrip propagation timein a cell, the so called RTT, so that the GP in such a case will bebased on the size of the cell.

As compared to the current situation, a prior art subframe (LTE TDD Type2) may only contain DwPTS and GP. This may lead to significantefficiency losses, which can be avoided by the subframe of theinvention, since the invention allows for the use of part of thesubframe for uplink transmissions as well, i.e. the UpPTS part.

The total sum of the durations of the DwPTS, UpPTS and the GP constitutethe total subframe length, which is a difference as compared to the LTETDD frame structure type 2, and also as compared to the frame structureused in systems which use TD-SCDMA, Time Division Synchronous CodeMultiple Access.

Another improvement of the invention over the LTE TDD frame structuretype 2 is that the length of the different parts can be varied, forexample according to the need for a guard period based on the maximumroundtrip propagation time in the cell and requirements of co-existencewith co-located or adjoining cells of other systems, as well as a needto adapt the capacity need between UL and DL on a finer scale thanpreviously possible.

Turning now to the LTE TDD frame structure type 1, a difference withregard to the inventive subframe is that part of the subframe of theinvention may be used for uplink transmission. Currently, in the LTE TDDframe Type 1, a subframe allocated to DL may only be used for DLtransmission, and may possibly also contain an “idle” guard part, i.e. apart that is not used for transmissions. A difference between thesubframe of the invention and the LTE TDD Type 1 frame is thus that inthe subframe of the invention, UL data can be transmitted as well.

The subframe of the invention is suitably placed following a period ofDL subframes and before a period of UL subframes, i.e. at a transitionfrom DL to UL. The DL part of the inventive subframe is in such anapplication placed first in the subframe.

In another embodiment, the subframe of the invention can be placedfollowing a period of UL subframes and before a period of DL subframes,i.e. at a transition from UL to DL. The UL part of the inventivesubframe is in such an application placed first in the subframe.

Hence, at least two of the three parts of the inventive subframe can bevaried to fit the system needs, since, if two parts are varied, thethird part will naturally be determined by what is left over of the subframe.

If the guard period, the GP, is one of the parts which is varied, it maybe varied with respect to at least one of the following parameters:

-   -   1. Interference from or with other cells in the same system, or        other cells in other adjoining or co-located systems,    -   2. The size of the cell, which determines the maximum        propagation round trip time, RTT, in the cell,    -   3. The modulation scheme used for traffic in the cell.

In case 1 above, i.e. when the guard period is varied with respect tothe interference from or with other cells in the system, the duration ofthe GP may suitably be determined so that it is adapted to be at leastequal to the propagation time of signals from or to at least onecontrolling node in another cell in the system.

Suitably, at least one of the parts for uplink traffic, downlinktraffic, and guard period is varied freely, i.e. without discrete steps,according to the needs of the system. However, in case 3 above, i.e.when the duration of the guard period is varied according to themodulation scheme used for traffic in the cell, if the system is onewhich uses an OFDM modulation method, Orthogonal Frequency DivisionModulation, then at least one of the uplink traffic, UpPTS, and downlinkparts, DwPTS, may be given a duration which corresponds to an integeramount of OFDM symbols in the modulation method. Suitably, the UpPTS andDwPTS are given a length of 1 or 2 OFDM symbols, although other OFDMsymbol lengths can also be envisioned within the scope of the presentinvention.

Hence, the invention will facilitate harmonization of the two framestructures in present day LTE TDD systems into a single frame structurewhich is harmonized with LTE FDD frame structures, which will bebeneficial at the present stage in 3GPP standardization, or at a laterstage, as LTE evolves into the so called IMT (International MobileTelecommunications) Advanced.

The invention also solves some drawbacks of present day LTE solutions,namely in that it:

-   -   allows for a finer granularity when it comes to allocating        resources to UL and DL, as well as allowing for increased        flexibility when it comes to creating guard periods.    -   Allows for increased flexibility when creating UL and DL period        lengths, which is beneficial from a perspective of co-existence        with TD-CDMA systems, as well as with TD-SCDMA and WiMAX        systems.

FIG. 5 shows a rough flow chart of a method 500 of the invention. Stepswhich are options or alternatives are shown with dashed lines.

As has emerged from the description above, the method of the inventionis intended for use in a cellular communications system, in whichtraffic in a cell is sent in frames, and where each frame comprises afirst number of subframes.

A second number of the subframes are available for at least eitheruplink or downlink traffic, and as shown in step 510, at least one ofsaid second number of subframes is made to comprise at least threeparts, as shown in step 515, as follows:

-   -   One part which is utilized for uplink traffic, step 520,    -   One part which is utilized for downlink traffic, step 525,    -   One part which is utilized as a guard period, step 530.

The guard period part of step 525 is scheduled between the uplink andthe downlink parts, and as shown in step 532, the duration of at leasttwo of the three parts of steps 520, 525 and 530 may be varied to fitthe current system need.

As indicated in step 540, the method of the invention may suitably beapplied to a TDD-system, Time Division Duplex, i.e. a system with anunpaired spectrum, so that uplink and down link traffic in at least afirst plurality of cells in the system occur during different subframes,but on the same frequency.

However, as indicated in step 535, the method of the invention may alsobe applied to a half duplex FDD-system, Frequency Division Duplex, sothat uplink and down link traffic in at least a first plurality of cellsin the system occur during different subframes, and on differentfrequencies.

As shown in step 550, in one embodiment of inventive method, the guardperiod is one of said at least two of three parts, and the guard periodis varied in duration with respect to at least one of the followingparameters:

-   -   Interference from or with other cells in the same system, or        other cells in other adjoining or co-located systems,    -   The size of the cell, which determines the maximum propagation        round trip time, RTT, in the cell,    -   The modulation scheme used for traffic in the cell.

As shown in step 545, the guard period may also be varied with respectto the interference from or with other cells in the system, so that theduration of the guard period is adapted to be at least equal to thepropagation time of signals from at least one controlling node inanother cell in the system.

In one embodiment, as shown in step 560, the inventive method may beapplied in a system in which an OFDM modulation method, OrthogonalFrequency Division Modulation is used in at least one of the uplink anddownlink directions, and at least one of the uplink traffic and downlinkparts in said second number of subframes is given a duration whichcorresponds to an integer number of OFDM symbols in the modulationmethod.

Also, in a further embodiment of the method of the invention, thesubframe which is made to comprise at least three parts is interposedafter a subframe used for downlink traffic and is followed by a subframewhich is used for uplink traffic, with the downlink part being first insaid subframe.

However, in an alternative embodiment, the inventive subframe which ismade to comprise at least three parts is interposed after a subframeused for uplink traffic and is followed by a subframe which is used fordownlink traffic, with the uplink part being first in said subframe.

As shown in step 570, the method of the invention can be applied to anLTE system, Long Term Evolution.

FIG. 6 shows a block diagram of some parts of a first transceiver 600which is intended for use as a controlling node in a system of theinvention. With retained use of the exemplary LTE terminology, thetransceiver 600 will be referred to as an eNodeB. Since the eNodeB ofthe invention works basically according to the method which has beendescribed above, all of the details of the operation of the eNodeB willnot be repeated again here.

The decision or decisions regarding the details of the inventivesubframe 420, such as, for example, the duration of the three parts, theDwPTS, the GP and the UpPTS, can be decided in a number of differentways in a system of the invention. For example, the decision can betaken by the operator of the system, and simply forwarded to the eNodeB600. In order to open for this possibility, the eNodeB will compriseinput means 610 for receiving such decisions. The input means 610 aresuitably an interface towards another, “higher” node in the system, viawhich the eNodeB communicates with the system.

The decision from the operator of the system can also be to let theeNodeB decide the particulars of the inventive subframe in a more orless autonomous manner. For example, the eNodeB can be instructed todecide the particulars of the inventive subframe in a completelyautonomous manner, based on, for example, interference measurementswhich the eNodeB carries out. To open up for such a possibility, theeNodeB comprises measurement means 620, which can measure theinterference in the cell.

A third possibility is that the operator instructs the eNodeB to decidethe particulars of the inventive subframe in a semi-autonomous manner,e.g. based on interference measurements, but with certain conditionswhich are laid down by the operator, such as, for example, that theduration of one of the three parts, the DwPTS, the GP and the UpPTS, maynot exceed or be shorter than a certain specified period of time.

Regardless of how the particulars of the inventive subframe are arrivedat by the eNodeB 600, the eNodeB 600 will comprise means 630 for takinga decision to arrive at these particulars. As shown in FIG. 6, thisdecision making means 630 can receive information both from the inputmeans 610 and the measurement means 620. The decision means 630 willalso suitably carry out the actual setting of the particulars of theinventive subframe in the eNodeB 600. Suitably, the decision and settingmeans will comprise a microcomputer or some similar computing component.

In addition, the eNodeB 600 will also need to communicate the details ofthe inventive subframe to the UEs in the cell, as well as to UEs whichare on their way in to the cell, i.e. UEs in a so called “hand overprocedure”, and also to UEs which are turned on in the cell, i.e. UEswhich have entered the cell with their power turned off, and which areswitched on in the cell, For this reason, the eNodeB 600 is shown ascomprising communication means 640, which will suitably comprise atransmitter and an antenna, which are normally comprised in an eNodeBfor communication with the UEs in a cell.

Thus, the information regarding the inventive subframe which the eNodeBcommunicates to the UEs in the cell will comprise the duration of thedifferent parts of the subframe of the invention, i.e. the DwPTS, GP andUpPTS. One preferred method of signaling this information to the UEs ina cell is to use the channel known as BCH, the “Broadcast Channel”,although the information may in principle be communicated to the UEs viaother control channels in the system.

FIG. 7 shows a block diagram of some parts of a second transceiver 700of the invention, which is intended for use as a user terminal(telephone/portable computer etc) in a system of the invention. Withretained use of the exemplary LTE terminology, the transceiver 700 willbe referred to as a UE, “User Equipment”. Since the UE of the inventionworks basically according to the method which has been described above,all of the details of the operation of the UE will not be repeated againhere.

The UE 700 of the invention is, as is indicated in FIG. 7, equipped withmeans for receiving instructions from the eNodeB of the cell regardingthe duration of the three parts of the inventive subframe, i.e. theDwPTS, the GP and the UpPTS. Suitably, these instructions are receivedvia the same means as other communication from the eNodeB, i.e. via areceiver and an antenna of the UE.

The instructions which are received from the eNodeB are then processedby the UE, i.e. the UE is set to those values of the DwPTS, GP andUpPTS. This is done by means 720 for setting or reconfiguring the DwPTS,GP and UpPTS in the UE. The setting and/or reconfiguration means willsuitably comprise microcomputer or some similar computing component.

In conclusion, the invention facilitates harmonization of the two framestructures in LTE for TDD into a single frame structure which can begiven a subframe duration of 1 ms. In addition, the invention alsosolves a number of drawbacks of present solutions, for example;

-   -   Allows for increased flexibility in creating of UL and DL period        lengths, which is beneficial from co-existences perspective with        TD-CDMA as well as TD-SCDMA and WiMAX.    -   Allows fine granularity when it comes to allocating resources to        UL and DL, as well as increased flexibility when creating guard        periods,

Another principle which may be used is so called half duplex FDD,Frequency Division Duplex, in which uplink and downlink transmissionsfrom one and the same terminal in the system occur on differentfrequencies and during different intervals in time, such as thepreviously mentioned subframes. The invention can also be applied insuch a system, i.e. a half duplex FDD system.

The invention is not limited to the examples of embodiments describedabove and shown in the drawings, but may be freely varied within thescope of the appended claims.

1. A method for use in a cellular communications system, in which systemthe traffic is sent in frames, each frame comprising a first number ofsubframes, with a second number of said subframes being available for atleast either uplink or downlink traffic, according to which method atleast one of said second number of subframes is made to comprise atleast three parts, as follows: One part which is utilized for uplinktraffic, One part which is utilized for downlink traffic, One part whichis utilized as a guard period, with said guard period part beingscheduled between the uplink and the downlink parts, and according towhich method the duration of at least two of said three parts may bevaried to fit the current system need.
 2. The method of claim 1, appliedto a TDD-system, Time Division Duplex, i.e. a system which uses anunpaired frequency spectrum, so that uplink and down link traffic in atleast a first plurality of cells in the system occur during differentsubframes,
 3. The method of claim 2, in which the uplink and thedownlink traffic occur on the same frequency.
 4. The method of claim 1,applied to a half duplex FDD-system, Frequency Division Duplex, so thatuplink and downlink traffic for one and the same user in at least afirst number of cells in the system occur during different subframes,and on different frequencies.
 5. The method of claim 1, according towhich the guard period is one of said at least two of three parts, andaccording to which method the guard period is varied in duration withrespect to at least one of the following parameters: Interference fromor with other cells in the same system, or other cells in otheradjoining or co-located systems, The size of the cell, which determinesthe maximum propagation round trip time, RTT, in the cell, Themodulation scheme used for traffic in the cell.
 6. The method of claim5, according to which the guard period is varied with respect to theinterference from or with other cells in the system so that the durationof the guard period is made at least equal to the propagation time ofsignals from at least one controlling node in another cell in thesystem.
 7. The method of claim 1, according to which the system is onein which an OFDM modulation method, Orthogonal Frequency DivisionModulation is used in at least one of the uplink and downlinkdirections, and in which at least one of the uplink traffic and downlinkparts in said second number of subframes is given a duration whichcorresponds to an integer number of OFDM symbols in the modulationmethod.
 8. The method of claim 1, according to which said subframe whichis made to comprise at least three parts is interposed after a subframeused for downlink traffic and is followed by a subframe which is usedfor uplink traffic, with the downlink part being first in said subframe.9. The method of claim 1, according to which said subframe which is madeto comprise at least three parts is interposed after a subframe used foruplink traffic and is followed by a subframe which is used for downlinktraffic, with the uplink part being first in said subframe.
 10. Themethod of claim 1, applied to an LTE system, Long Term Evolution.
 11. Atransceiver for use as a controlling node in a cell of a cellularcommunications system, the transceiver being adapted to send and receivetraffic in frames, with each frame comprising a first number ofsubframes and with a second number of said subframes being available forat least either uplink or downlink traffic, the transceiver beingequipped with means for sending and receiving at least one of saidsecond number of subframes in at least three parts, as follows: One part(UpPTS) which is utilized for uplink traffic, One part (DwPTS) which isutilized for downlink traffic, One part (GP) which is utilized as aguard period, the transceiver being equipped with means for schedulingsaid guard period part between the uplink and the downlink parts, andfor varying the duration of at least two of said three parts to fit thecurrent system need.
 12. The transceiver of claim 11, additionally beingequipped with means for receiving information from an external source inthe system regarding the varying of said three parts.
 13. Thetransceiver of claim 11, being equipped with means for transmitting tousers in a cell of the system information regarding the duration of saidthree pats.
 14. The transceiver of claim 10, adapted to be used in aTDD-system, Time Division Duplex, i.e. a system which uses an unpairedspectrum, so that uplink and downlink traffic occur during differentsubframes.
 15. The transceiver of claim 14, adapted to be used on thesame frequency for both uplink and downlink traffic.
 16. The transceiverclaim 11, adapted to be used in a half duplex FDD-system, FrequencyDivision Duplex, so that uplink and downlink traffic for one and thesame user occur during different subframes, and on differentfrequencies.
 17. The transceiver of claim 10, in which the guard periodis one of said at least two of three parts, and is equipped with meansfor varying the guard period in duration with respect to at least one ofthe following parameters: Interference from or with other cells in thesame system, or other cells in other adjoining or co-located systems,The size of the cell, which determines the maximum propagation roundtrip time, RTT, in the cell, The modulation scheme used for traffic inthe cell.
 18. The transceiver of claim 17, adapted to vary the guardperiod with respect to the interference from or with other cells in thesystem, so that the duration of the guard period is adapted to be atleast equal to the propagation time of signals from at least onecontrolling node in another cell in the system.
 19. The transceiver ofclaim 11, being adapted for use in a system in which an OFDM modulationmethod, Orthogonal Frequency Division Modulation is used in at least oneof the uplink and downlink directions, being equipped with means forgiving at least one of the uplink traffic and downlink parts in saidsecond number of subframes a duration which corresponds to an integernumber of OFDM symbols in the modulation method.
 20. The transceiver ofclaim 11, which comprises means for interposing said subframe which iscomprised of at least three parts after a subframe used for downlinktraffic and before a subframe which is used for uplink traffic, with thedownlink part being first in said subframe. 21-23. (canceled)